In industrial applications such as the manufacture of polypropylene (PP) pallets, a highly demanding balance of stiffness (rigidity) and impact strength is required. For example, the pallet needs to respond in a ductile fashion (i.e., no brittle failures) when impacted by a heavy object, e.g., three orders of magnitude heavier than the pallet (sled) at a defined speed (referred to as the “sled impact test”) so that the pallet can be reused. At the same time, no cracking (brittle failure) is acceptable, when the pallet is dropped from a relatively high height (e.g., 5-20 feet) on one of its corners (referred to as the “corner drop impact test”). In addition, the ICP composition needs to possess enhanced creep resistance, so that a pallet having a weight of e.g., about two orders of magnitude higher than the pallet weight layered on top of it for an extended period of time (e.g., 20-40 days) at a relatively high temperature (e.g., 40-50° C.) does not deflect above a certain strain level (e.g., deflection less than a certain pre-defined amount).
The latter may be referred to as a “pallet deflection test.” Alternatively, pallet tests related conceptually to the tests noted above are described in ASTM D1185-98A. ASTM D1185-98A does not disclose specific values/requirements for creep deflection, as they can be application dependent.
In addition, the melt rheology of the material should be such that reduced cycle times and reduced injection pressures can be achieved during the injection molding process. Therefore, a higher melt flow rate (MFR) ICP is desired from a process performance viewpoint to reduce cycle time and increase productivity.
In order to meet such demanding product requirements, a proper molecular design of the ICP material is important. Typically, when the material has increased rigidity (e.g., passes the deflection test), it decreases in impact resistance (e.g., fails the drop impact and/or sled impact tests), since rigidity and impact strength normally work against each other. While a higher MFR material (i.e., one with a lower molecular weight) favors processability, it hurts the impact strength leading to brittle failures associated with the drop impact and/or sled impact tests. Therefore, designing the molecular architecture of a high MFR ICP that meets both rigidity and impact requirements is counterintuitive by nature.
U.S. Pat. No. 6,284,833 discloses reactor olefin polymer compositions comprising isotactic polypropylene as a continuous phase and an ethylene-propylene rubber (EPR) copolymer as a discontinuous phase having good paintability, and that are particularly attractive for use in the production of automotive trim and fascia. However, the disclosed EPR composition is C2(ethylene)-rich, comprising of 40-55% wt. C2. Such a composition may be detrimental for passing severe impact situations such as the sled impact test for the pallets (e.g., see comparative example IV in Tables 1 and 2, below). The compositions of the present invention are vastly different from these compositions, in that the rubber is C3(propylene)-rich (as approximated by the percent C2 in xylene soluble fraction (XS) of less than about 39%); this is an important element in combination with other molecular design characteristics in achieving optimum stiffness-impact balance, especially in the case of the pallet application and water-storm chambers.
U.S. Pat. No. 7,482,406 relates to a polypropylene impact copolymer-type composition, which requires a highly isotactic/crystalline matrix with percent mmmm (meso-pentads) homopolypropylene (HPP) xylene insolubles (XIS) greater than 98% to achieve good stiffness-impact balance. This is in contrast to the present invention, which preferably and surprisingly utilizes a less isotactic matrix (e.g., % mmmm XIS of about 96.4-97.8%) to achieve superior stiffness-impact balance performance.
U.S. Pat. No. 5,929,147 discloses an embrittlement-resistant polyolefin composition which is a blend of at least 80% by weight of a crystalline polymer, comprising either a propylene homopolymer or a random copolymer of propylene and either ethylene or C4-C10 1-olefins and less than 20% of an elastomeric copolymer. This is in contrast to the compositions of the present invention which contain at least 20% elastomeric component and are opaque (haze is typically greater than 70%).
U.S. Pat. No. 7,348,381 relates to compositions comprising a polypropylene homopolymer portion and an ethylene-propylene rubber (EPR) portion interspersed therein. The reference states that reduced molecular weight (with low intrinsic viscosity) of the elastomer modifier adversely affects the impact strength of the thermoplastic olefin composition (column 3, lines 3-8), contrary to the present invention.
U.S. Pat. No. 6,300,415 discloses propylene compositions comprising a propylene (PP) and a propylene-ethylene copolymer (RC) where the intrinsic viscosity ratio of the RC over that of the PP is in the range of 0.7 to 1.2. The composition of this invention comprises an intrinsic viscosity ratio of greater than 1.2. This reference also discloses the mathematical quantity defined as the product of the ratio of weight percent PP over weight percent RC times the ratio of the intrinsic viscosity of RC over the intrinsic viscosity of PP to be in the range 1-3; this quantity exceeds a value of 3 for the composition of this invention (Table 1). The compositions of U.S. Pat. No. 6,300,415 exhibit transparency, while the compositions of the present invention typically are opaque with haze values typically well above 70%.
Compositions of U.S. Pat. No. 5,973,078 are blends of olefin polymers, where one component is a high molecular weight branched polymer and the other component can comprise a heterophasic propylene-based polymer. Such compositions are suitable for producing high tenacity fibers and are unrelated to the making of molded articles with enhanced stiffness-impact balance, such as pallets and water-storm chambers. This is in contrast to the present invention, where no branched polymer is used as a blending component, with the polymer structure being substantially linear.
U.S. Pat. No. 6,943,215 relates to an impact-resistant polymer blend comprising (a) a crystalline polypropylene matrix having a weight average molecular weight, and (b) an at least partially crystalline copolymer impact modifier having a molecular weight lower than the weight average molecular weight of the crystalline polypropylene matrix, the impact modifier comprising propylene and ethylene and/or one or more unsaturated co-monomers, the modifier prepared using a non-metallocene, metal-centered, heteroaryl ligand catalyst. Such compositions are in contrast to the present invention wherein the weight average molecular weight of the EPR phase is higher than that of the HPP matrix, as reflected in the constraint Mw XS/Mw XIS of 1.05-1.5, as described below.
Increasing the MFR of the ICP resin to reduce injection molding process cycle time normally has a negative effect on the stiffness-impact balance of the material, causing it not to meet the final product requirements. Therefore, increasing the MFR typically sacrifices impact resistance, leading to undesirable brittle failures.
Conventional products for pallet applications are ICPs of less than about 7.5 MFR, suffering from high cycle time and high injection pressures that are detrimental for the injection molding tools from a mechanical point of view. Therefore, the low MFR ICPs (less than about 7.5) used in the prior art for pallet applications have a negative impact on cycle time and process efficiency, due to their high melt viscosity.